The present invention relates to a dispenser for applying cream-like solder on a substrate of a semiconductor device so as to solder terminals of an electronic element to a wiring pattern of the substrate.
In order to mount an electronic element such as an integrated circuit (IC) chip on a substrate of the semiconductor device, the terminals of the electronic element are soldered on the wiring pattern formed on the substrate. When a compact electronic element having a plurality of terminals is mounted on a small substrate at a high density, a cream-like solder having a low fluidity is used.
FIG. 5 describes a method for applying cream-like solder 40 which contains flux of 10% to 15%. The solder is applied on a plurality of lands 21 of a wiring pattern formed on a substrate 10 of a semiconductor device. The solder 40 is continuously applied on the lands 21 through a dispenser 30 having a cylindrical needle tube 31 with an opening 32 at the end thereof. Thus, a long strip of solder 40 is formed over the lands 21 as shown in FIG. 6. Thereafter, as shown in FIG. 7, terminals 51 and 52 of an electronic element 50 is mounted on the lands 21. When heated, the solder 40 is melted, thereby forming a meniscus of the solder because of surface tension thereof. As a result, the solder 40 is divided into individual blocks 41 and 42 on each land, so that each of the lands 21 are insulated from one another.
However, since the solder 40 is dispensed through the circular opening 32, a large quantity of solder 40 is apt to be accumulated on the land to form a spherical strip having a large height H as shown in FIG. 6. The terminals 51 and 52 are accordingly buried in the blocks 41 and 42 as shown in FIG. 7, so that the solder 40 may spread out of the lands 21. consequently, bridges which cause a short-circuit between the lands 21 are formed.
Moreover, the strip of solder 40 shown in FIG. 6 is not sufficiently spread to cover the entire surface area of each land. Namely, the width W of the strip is far smaller than the length L of the lands 21, so that a large bare area is formed on the land. Since the bare surface is not cleansed by the flux included in the solder, an oxide film remains on the land. As a result, the land 21 has poor wettability so that juncture area between the terminal of the electronic element and the land is small, resulting in decrease of the juncture strength and increase of the electrical resistance therebetween.
Furthermore, the juncture strength of the solder decreases as the quantity of solder on the juncture area increases. Since the solder 40 is accumulated to form a mound as shown in FIG. 7, the quantity of the solder at the juncture area is large. Thus, the juncture strength of the portions 41 and 42 tends to be further decreased.